Two stage filtration of sewage



Jan. 1, 1952 J, GUNZ 2,580,764

TWO STAGE FILTRATION OF SEWAGE Filed Oct. 5, 1947 IN VEN TOR. 21 b Patented Jan. 1, 1952 STATES OFFICE f 2,5so,.7c1 "f TWQ STAGE" SEWAGE Johanna Gunz, Tucson, Arizl assignon tolnfilc'o Incorporated, Delaware i no a.-.cornoration of Application October 3, 19,417; SerialjNQ; 777,613 5 1, I Thisinventionrelatesto thetreatment oi sewerage nd the like. lay-filtration throu h icklin filters P 'It is; an object; or this inventlon to provide an --improved sewage, treating system including filtering sewagezthrough a plurality of filters. Anotherobject of my invention is to provide a tricklinggfilter' planter system wherein the filters areconnected inamanner to equalize theorganic load on-tboth, and-.to; maintain both at highest -efiiciency; r

Another object oi-the'invention is-to provide two, stage'filtration-of sewage with vthe filters of each stage serving simultaneously as primary and assecondary filtersn Another object istoprovidea tri l plant-combining the. advantages: of parallel and w stage-filtration. r

Theseqand other obiectsiot theinvention will become apparent upon consideration ofthe de- --scrip-tion and claims which follow. 7

g E-Xceptin small plants whereone filter is-sufii cient for treating the entire volume of'sewage, .tricklineiifilter plants are usually laid out either parallel or series, or-stages; as they are sometimes;- called. 7 In the first case, theincoming ...raw---sewage, usually after removal of settleable ,solidsain apreliminary treatingdevice,-such-.as a clarifier, issplit into several equal portions, each filter-being i dosed with-one portion. Where recircu1ation-.-ispractiseddn-smh asystem,each

filter-receives-in addition to the raw sewage some of its, own ,efil-uenttor repeated treatment Each filter thus treatsa distinct portion of the incom- Jng sewage,- either once orrepeatedly, but'no portionof thesewageis treated onseveral filters and the efiluent ofyeachfilter goes-to a final settler which maybe common to a group or to all of the filters Obviously, insuch aisystem of filters operating inparallel, the work to be ,done is split into equalportions, each filter doing the same amount. -and ,kind .of. work, but nointeraction Or cooperation of the :filter exists. otherwise,

It is, generally maintained that Where 'a high i rad of, purificat on, is. requ red, this can be ,ac-

.complished more .economi-cally. by filtering the sewage in sequence through filters of two stages rather than by, filtration in parallel; Two stage filtration, with or without recirculation, has Icecome a widely accepted practice, In suchasystom, ,thsjfiltet Ofthe firs t ge is dosed with the zran's asa Thes fiuen Qii hepr marr fi t r is iflor. thoutin rme iat settl ng, 9 g?! jgfil i-o,'llhceifiuenmdithesec- -'or'idaryfilter is passed to a final clarifier or the Y Where .l ceeuaeon. of filter enuenn is practised, thefirstfilter-in such a two. stage system mayalso be;dosed, in addition to the raw sewage,

- with some, portion of its. own eflluent, or oithe secondary efliuent,orla mixture of, both. .Simi- -larly, thesecond-a'ry. filter, beside receiving the recyclediportion'ofirits, own eflluent.

7 tion; they are operatedin units of two, primary and secondary, with, the units operating in .par-

- allel; However,for-simplification only one set of filters has-been mentioned, asthe problem ,to be discussedis thevsame, Whetherone or-several sets Thus-ordinarily, in prior arttwo stagefiltration systems, the secondary filter did not receive --raw -settled,-sewage,- but only filteredsewage from 20,

the primary filter, and sometimes also twice filtered sewage in othe form of it own effluent, the

rawrsewage goingionlyto the primary filter. Obviously, theorganic-load ton the filters of the two "stagesof such a systemis-not equal. The purificationof raw-sewageputs a far heavier load on the; primary filter than the further purification ofpre-treated sewage on the secondary, The

workytohe-performed by the filters di-fiers not :o-nlyinq-uantitybut also in quality.

- lt wasrecognizedin the art that the filters in such a system are not utilizedv in the best and dosing-thefilters. -alternately as-prima-ry filter and as secondary fullest manner; In order to equalize the organic loadon-the filters and to enable-each -of them to Workunderrcptimurn conditions, it has been suggested to periodically alternate the sequence of Thus each filter would serve filter on-the-theory that in such a system each filter, during the; period itserves as secondary filter can recover from the strain of the organic load putson it duringthe period it serves as primany filter.- T'he primary filter; which take the reate tr-portiongcrthe or nic load, will thus eriodical y -be; reconditioned th t n it ou otoperation,

Experience withQthissystem showed that the filterscouldnotreadily recover during their period orrelative'restas secondary filters, unless ,theywere suppliedby the primary filter, with a fairly. good, eflluent In other words, the secondaryofiltennin order to rec ver or servic as r.

mam ,cou di-do very, little orls, and; the. pur fication had, toheef lected tothe greatest part by h prim.a,rng=lf;the load @was too. heavy: for th ip may to 'g'lve a'good eliluent thesecondary would as a combined primary and secondary filter. and each filter receives raw sewage and also filtered sewage, whereby the organic load is equalized. Inthis respect the system is similar to, and has the advantages of, parallel filtration. However, in my system the filters are not connected in parallel, but in stages, or sequence. While'in parallel filtration each portion of the sewage is treated only on one filter and there is no interaction and cooperation of the filters in treating individual portions of sewage, in my system the sewage is treated by cooperation of the filters of both stages, material from each filter going to the other for further treatment. On the one hand there is a through flow of sewage from a primary clarifier or the like to a first filter, thence to a second filter and thence to a final clarifier or the like. In this respect my system is similar to and has the advantages of a conventional two stage plant. At the same time, however, there is also a fiow of raw sewage from the clarifier directly to the secondary filter and a return of material treated on the secondary filter to the first filter. In this manner, the organic load on each filter is about equal, each receiving partly raw sewage and partly material pretreated on the other, thus combining the functions of primary filter and of secondary filter.

My invention will be more readily understood from a consideration oi the drawing wherein Figures 1 through 3 show diagrammatically three preferred fiow systems embodying my invention.

As shown in Figure 1, raw sewage entering through an inlet I is first passed into a clarifier l I for removing settleable solids prior to its treatment on the trickling filters. In some instances, only a grit chamber or a screen may be needed and the clarifier dispensed with, but ordinarily, and especially in larger plants, a clarifier will precede the biological treatment. The settled raw sewage leaves the clarifier through a conduit l2. The fiow is split and one portion is passed into each of the two trickling filters, l3 and M, respectively, as through conduits l5 and I6. As shown, the eilluent from filter I3 (which for convenience may be called the first filter although in my system the two'filters do not have the usual characteristics distinguishing first and secondary filters in conventional two stage filtration) is passed to the other (or second) filter i4 through a conduit 20. The eiliuent from the second filter is divided, one portion being returned to the inlet of the first filter through a conduit 2| and another portion being discharged through a conduit 22 to a final clarifier 23 whose overflow goes to waste through a conduit 24. Obviously, if desired. each filter may also additionally be dosed with some of its own effluent, as is well known in the art, but this is not essential in my system and in view of the additional piping and pumps needed therefor is less desirable. The several conduits will be provided with the necessary valves and pumps. not shown. It may be mentioned that for simplification, the

underflow of the clarifiers is not shown in the drawing, as this can be disposed of in any known conventional manner which forms no part of the invention.

The system as described provides each filter with raw sewage and with sewage that has been treated on the other. Obviously, with equal portions of .raw sewage going to both filters, the hydraulic load on the second filter will be greater than on the first, as it receives the same amount of raw sewage and additionally everything that goes through the first filter. The organic load. however, which is commonly expressed in terms of B. O. D. (biochemical oxygen demand) applied, will not be proportionally greater but will be approximately the same on both filters, for the organic load stems mainly from the raw sewage. It is known, and has been proven convincingly by the experiments with alternate double filtration, that a good eiiluent from a first stage filter does not put a substantial organic load on the secondary filter. The average B. O. D. reduction by a conventional low rate trickling filter is generally assumed to be about 70 percent. For example, Metcalf and Eddy, American Sewerage Practice, Volume 3, state (page 486) that, in general, the reduction in B. 0. D. by trickling filters is 60 to percent. The effluent from my first filter is much better than usual first stage filter eiiluent, as due to the division of the raw sewage between the two filters the load on the first is halved. This lesser amount of raw sewage is diluted with recirculated sewage which--to some extent-has been twice previously filtered.

While I have spoken of equal portions of raw sewage going to both filters, this proportion is not sharply critical. However, to obtain the advantages of my invention, a very substantial portion of the raw sewage should be applied to the secondary filter.

The equalization in the organic load provided by my system as compared with a conventional two stage trickling filter system will be readily seen by considering that in a conventional two stage plant the first filter receives percent of the B. O. D. of the raw sewage, while the second filter, on the basis of '10 percent B. O. D. reduction by the first filter, receives only 30 percent. Taking the same figure of 70 percent B. O. D. reduction for my system and disregarding for the moment the recirculation from the second t0 the first filter, then, with equal amounts of raw sewage going to the two filters, the first filter of my system receives 50 percent of the B. O. D. (applied to it in the raw sewage), and the second filter receives 50 percent of the B. 0. D. (applied to it in the raw sewage), plus 15 percent of the B. O. D. (applied to it in the eilluent from the first filter). Thus, without any recirculation from the second to the first filter, there is a difference of only 15 percent in the load applied to the two filters, as compared with a difierenceof 70 percent in a conventional two stage filter system. When applying the initial B. O. D. of the raw sewage at the rate say of 55 percent to the first filter and 45 percent to the second, instead of the equal amounts of the above example, the difi'erence is only .5 percent. The difierence is further reduced by the recirculation. Obviously, any amount of B. O. D. returned from the second to the first filter is reduced thereon, and only a small percentage of this recirculated B. O. D. is passed on to the second filter. When considering these simple arithmetics of my system,

assume sired degree of equalization-under any set of circumstances can be readilycom putedt;

My system? eliminates completely-the danger of some portion of untreated sewage being discharged with the eilluent. Every portionof sewage entering the system has tob'e treated;at-'-least once before it can leave the system. Someof the raw-sewage entering the second filter willflbe treated only once and-thenbe dischargedto-the final clarifier.- Butythis is not objectionable, as the raw. sewage entering the second filter isdil-u-ted with a'larger' quantity" of eminent from the first filter which is rich in active bacterlalgrowth and which, as stated-above, is partly -on;i-ts second pass throughboth filters.

It'i's-one of'the advantages; of my 'system that each of the two filters is continuously seeded with fresh active bacteria from the other, so that the bacterial population of both filters is constantly maintained active. Also by this continuous exchange of bacterial growth between the filters, the type of bacteria on both will become substantially uniform, so that each filter is enabled to do the same amount and kind of work as the other. The raw sewage in my system, is acted upon by a much greater quantity of active bacter a which are in a better condition than when all settled raw sewage is treated first on the primary filter and only filtered sewage treated on the secondary filter.

The system of Figure 2 is essentially the same as that of Figure 1 with the one exce tion that in this case the recirculated portion of the secondary filter efiluent is not brought back directly to the first filter but is dischar ed in o the primary clarifier as through conduit 21a. In general th s is less desirable, as most of the active bacterial growth contained in the fil er efil ent will settle out in the clarifier and will thus not be available for seedin the filter. However, in some cases, as where heavy slugs of bacterial slime are discharged from the second filter, it may be of advantage to settle these out prior to applying the recirculated efiluent to the first filter. It will also be noted that n this case only a portion of the recirculated effl ent from the second filter will go to the first filter, while the other will return to the second filter alon with its portion of raw sewage. In this system. therefore. a greater proportion of the final eill ent will have been treated only on the second filter, which is less desirable.

Consideration of Figure 3 shows that th s system in its essential features is the same as that of Figure 1. Here, however, all of the secondary filter efiluent is discharged into the final clarifier and the sewage which is recirculated is taken from the final clarifier, preferably from some intermediate portion thereof. As shown, the recirculated sewage may be returned directly to the first filter through conduit 2 lb and'2lc similarly to the system described in connection with Figure 1, or first be sent to the primary clarifier through conduits 2!!) and Zld, smilarly to the system of Figure 2. This system is less-desirable thanthe preferred system of Figure 1, as part of the active bacterial growth contained in the efiluent ofithe second filter will settle out and can thus not be brought back on the first filter for seeding. Also, the'inaterial taken from such a clarifier is likely 'm i be somewhat stal'e fand what bacterial 7 population it contains will not be in'active'cond-itio'rr.

It will be seen that by changing the manner of operation of two stage "trickling fi-lter -plzm ts I secure highly-improved resu-ltsz- A- special advantage of my system is that it can be incorporated? and itsresults obtained in, a conventional two stage-trickling filter plant-= by making relatively small structural changes involving only l-ittl'e-expense.=-

It will' be understood that the figures sh'ow simplifie'd and diagrammatic fiow sheets from which all conventional parts which are not essential to the understanding of my systemhave -been'omitted: The combination-of such parts; as for instance the clarifierunderfiow, or the conventional recirculationfrom' outlet to inlet of'the same filter, with my-ficw system, is: to b'econstrued as within the scopeof-the invention.

I-cl'aim: I 1 A sewage treating system comprising -arraw sewage inlet, a first trickling filter and a second trickling filter, conduit means connecting said raw sewage inlet and said first and second filters in the sequence recited, conduit means for delivering raw sewage from said raw sewage inlet to said second filter, an outlet to waste from said second filter only, and conduit means leading from the outlet of said second filter and discharging to said first filter.

2. A sewage treating system comprising a primary clarifier, a first biological filter, a second biological filter, and a final clarifier, piping connecting said clarifiers and filters in the sequence cited, a raw sewage inlet into the primary clarifier, and'a treated sewage outlet from the secondary clarifier, characterized by conduit means for conveying a portion of the raw sewage coming from the primary clarifier to the second filter, and conduit means for returning a portion of the efiiuent from the second filter to said system upstream of the first filter.

3. In a system for treating sewage by two stage filtration comprising a clarifier, a first trickling filter and a second trickling filter, means for continuously passing a portion of raw sewage from the clarifier to said first filter for a first treatment therein and another portion to said second filter for a first treatment therein, means for passing the unsedimented effluent of said first filter to said second filter for further treatment therein along with the raw sewage receiving its first treatment on said second filter, and means for returning a portion of the sewage treated on the second filter to the system upstream of the first filter for further treatment on said first filter along with the raw sewage receiving its first treatment thereon, and means leading from said second filter for discharging the efliuent of said system to waste.

4. In a sewage 1 treating plant comprising a first clarifier havingfa raw sewage inlet and a clarified raw sewageoutlet, a first biological filter, a second biological filter, and a second clarifier having an inlet and an outlet to waste, an inlet for clarified raw sewage into each of said filters, said clarified raw sewage inlets being connected to said outlet from said first clarifier, an outlet from each of said filters, the outlet from the first filter being connected 'to said inlet to the second filter, and the outlet of said second filter being connected to the inlet of said second clarifier and to the inlet of said first filter.

5. In the biological filtration of sewage, a method of equalizing the organic load and the bacterial population on a first and a second trickling filter arranged in sequence which comprises dosing each filter with a substantial portion of the incoming sewage to be treated, seeding the secondary filter with fresh bacterial from the first filter by dosing it additionally with the unsedimented effluent of the first filter, seeding the first filter with fresh bacteria from the second filter by dosing it additionally with an unsedimented portion of the efiluent of the second filter, and withdrawing the balance of the effluent of the second filter.

6. In a sewage treating plant comprising a raw sewage inlet, a first biological filter, and a second biological filter, conduit means connecting said raw sewage inlet to each of said filters, means including a conduit for passing unsedimented effiuent of said first filter over said second filter, means including a conduit for passing a portion of the eilluent of said second filter oversaid first filter, and means for withdrawing the balance of the eflluent 01 said second filter to waste. JOHANNA GUNZ.

REFERENCES CITED The following references are of record in the file 01 this patent:-

UNITED STATES PATENTS Number Name Date Re. 22,144 Ward July 21, 1942 2,142,196 Landon Jan. 3, 1939 2,168,208 Jenks Aug. 1, 1939 2,355,760 Trebler Aug. 15, 1944 OTHER REFERENCES 

